1. Field
The present disclosure relates to wireless communications and, more particularly to apparatus and methods for timing tracking in a multiple receive antenna system.
2. Background
Certain types of communication systems such as those using Orthogonal Frequency Division Multiplexing (OFDM), for example, are highly sensitive to synchronization errors such as timing error and frequency error. In order for these types of systems to work properly, the receiver portion of a transceiver and the transmitter from which symbols are transmitted to the transceiver have to be synchronized, which includes timing and frequency synchronization. Ideally, synchronization and timing in the receiver portion should follow the transmitter. In OFDM systems, for example, timing tracking or synchronization involves finding the optimum sampling start position for a next OFDM symbol based on the current OFDM symbol channel. Thus, unless the correct timing is known, the transceiver cannot remove cyclic prefixes occurring between symbols at the right timing instant and correctly separate individual symbols before computing a Fast Fourier Transformation (FFT) of the sample for demodulating the symbol in the case of OFDM.
Accordingly, a goal of timing tracking in a system such as an OFDM system is to find the optimum sampling start position of a sampling window for the next OFDM symbol or channel given the current OFDM symbol or channel. The sampling position should be chosen such that intersymbol interference (ISI) as well as intercarrier interference (ICI), which are termed as the “effective interference” (EI), caused by the existing channel profile is suppressed and the signal-to-noise ratio (SNR) is correspondingly increased. Sources of EI can be classified into a number of types. The first type is static EI that, given the current channel profile, is a deterministic EI determined by the OFDM symbol structure, e.g., the length of a cyclic prefix. In a dynamic environment, however, channel time variation (new arrival paths that may appear in the future) and system timing error (e.g., sleep timing error) may also introduce EI. This EI can be typified as a dynamic EI that is random in nature and best described by probabilistic models. Another type of EI arises from channel fading, where faded channel taps may also affect the timing decision causing EI.
Pending U.S. patent application Ser. No. 11/264,532, which is incorporated herein, discloses methodology and apparatus for determining timing of a symbol in a wireless system that minimizes one or more of the three types of effective interference (EI) discussed above. By setting timing in a transceiver or a similar device to ensure minimization of the total effective interference (EI) (i.e., minimization of ISI and ICI and maximization of signal energy), performance of the transceiver to decode and demodulate is better optimized. In particular, the timing of a timing window is determined by taking into account at least one or more of the three different sources of effective interference (EI), namely static EI, dynamic EI and EI due to faded channel taps. This accounting may be accomplished by analytically deriving a composite EI function that combines the three types of EI into an effective EI channel density function.
Additionally, wireless communication systems may utilize a multiple receive antenna system in order to improve receive signal diversity and/or the capacity of the wireless system, such as an OFDM system. For instance, a multiple-input-multiple-output (MIMO) system, as one example of a multiple receive antenna system, employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. With the increase in the number of antennas in such systems, however, comes a resultant increase in system sensitivity to channel estimation errors.